查询以从单个查询获取多个结果或返回null

Question

我有一个假期表，其中包含以下字段

COLUMN_NAME         DATA_TYPE
-------------------------------
HOLIDAY_ID          NUMBER(10,0)
HOLIDAY_NAME        VARCHAR2(150 BYTE)
HOLIDAY_DATE        TIMESTAMP(6)
DAY_OF_WEEK         VARCHAR2(20 BYTE)
HOLIDAY_TYPE_ID     NUMBER

样本数据是

HOLIDAY_ID   HOLIDAY_NAME     HOLIDAY_DATE                      DAY_OF_WEEK     HOLIDAY_TYPE_ID
-------------------------------------------------------------------------------
305         Republic Day      26-01-17 12:00:00.000000000 AM    Thursday         1

306         Maha Shivratri    24-02-17 12:00:00.000000000 AM     Friday          1

我使用以下查询获取特定月份/年份的holiday_date： （QUERY）

select nvl((select to_char(holiday_date,'DD/MM/YY') holiday 
from admin_holiday where to_char(holiday_date,'DD-MON-YYYY') like '%-JUN-2016%' 
and holiday_type_id=1 and to_char(holiday_date,'Day') not like 'Sun%' 
and to_char(holiday_date,'Day') not like 'Sat%'),'0') holiday from dual

如果结果为空（即，月/年= JUN / 2016），但工作正常，但当月份包含多个假期（例如，月/年= OCT /）时，出现“单行子查询返回多个行”错误2016）

如何编写查询，以便如果查询返回多行然后显示那些结果，否则显示NULL结果为'0'？

[ 注意 ：假期按照印度日历]

Answer 1

您应该考虑将查询更改为此：

SELECT NVL(TO_CHAR(holiday_date,'DD/MM/YY'), '0') holiday
FROM admin_holiday 
WHERE TO_CHAR(holiday_date,'MON-YYYY') = 'JUN-2016' 
--WHERE TO_CHAR(holiday_date,'yyyymm') = '201606'
    AND holiday_type_id = 1 
    AND TO_CHAR(holiday_date,'Day') NOT IN ('Sun', 'Sat');

Answer 2

您需要将nvl函数应用于列和结果，如下所示：

select nvl(to_char(holiday_date,'DD/MM/YY'),'0') holiday 
from (select 1 as dummy from dual) d left join admin_holiday 
on to_char(holiday_date,'DD-MON-YYYY') like '%-JUN-2016%' 
and holiday_type_id=1 and to_char(holiday_date,'Day') not like 'Sun%' 
and to_char(holiday_date,'Day') not like 'Sat%'

附带说明：了解如何编写可查询的查询

Answer 3

SELECT NVL(TO_CHAR(holiday_date,'DD/MM/YY'), '0') holiday
FROM admin_holiday 
WHERE TO_CHAR(holiday_date,'MON-YYYY') = 'JUN-2016' 
--WHERE TO_CHAR(holiday_date,'yyyymm') = '201606'
    AND holiday_type_id = 1 
    AND TO_CHAR(holiday_date,'Day') NOT IN ('Sun', 'Sat')
union all
select '0' from dual where not exists(SELECT NVL(TO_CHAR(holiday_date,'DD/MM/YY'), '0') holiday
FROM admin_holiday 
WHERE TO_CHAR(holiday_date,'MON-YYYY') = 'JUN-2016' 
--WHERE TO_CHAR(holiday_date,'yyyymm') = '201606'
    AND holiday_type_id = 1 
    AND TO_CHAR(holiday_date,'Day') NOT IN ('Sun', 'Sat'))